There has been substantial interest in the preparation and characterization of compound semiconductors in the form of particles with dimensions in the order of 2-50 nanometers (nm), often referred to as quantum dots, nanoparticles, or nanocrystals. Interest has arisen mainly due to the size-related electronic properties of these materials that can be exploited in many commercial applications such as optical and electronic devices, biological labeling, solar cells, catalysis, biological imaging, light-emitting diodes, general space lighting, and electroluminescent and photoluminescent displays.
Two fundamental factors, both related to the size of the individual semiconductor nanoparticle, are responsible for their unique properties. The first is the large surface-to-volume ratio: as a particle becomes smaller, the ratio of the number of surface atoms to that in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material. The second factor is the change in the electronic properties of the material with size, e.g., the band gap gradually becomes larger because of quantum confinement effects as the size of the particle decreases. This effect is a consequence of increased carrier confinement giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than the continuous band of the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, because of the physical parameters, the carriers (i.e., electrons and holes) produced by the absorption of electromagnetic radiation (i.e., a photon) with energy greater then the first excitonic transition, are closer together than in the corresponding bulk (or macrocrystalline) material, so that the coulombic interaction cannot be neglected. This leads to a narrow bandwidth emission, which is dependent upon the particle size and composition. Thus, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and, consequently, the first excitonic transition (i.e., the bandgap) increases in energy with decreasing particle diameter.
Among the most studied semiconductor quantum dot materials have been the chalcogenide II-VI materials, namely zinc sulfide (ZnS), zinc selenide (ZnSe), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe). Reproducible quantum dot production methods have been developed from “bottom-up” techniques, whereby particles are prepared atom-by-atom, i.e. from molecules to clusters to particles, using wet chemical procedures. The coordination about the final inorganic surface atoms in any nanoparticle may be incomplete, with highly reactive non-fully coordinated atomic “dangling bonds” on the surface of the particle, which can lead to particle agglomeration. This problem may be overcome by passivating (e.g., capping) the bare surface atoms with protective organic groups.
Single-core semiconductor nanoparticles, which generally consist of a single semiconductor material along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to non-radiative electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface. FIG. 1A schematically depicts an indium phosphide (InP) single-core nanoparticle 100 with a core 110 including InP and an organic passivation layer 120. The hydrocarbon chains of passivation layer 120 promote monodispersity of a group of nanoparticles in solution.
One method to eliminate defects and dangling bonds is growth of a second inorganic material, having a wider bandgap and small lattice mismatch to that of the core material, epitaxially on the surface of the core particle to produce a “core-shell” nanoparticle. Core-shell nanoparticles separate any carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. Small lattice mismatch between the core and shell materials also minimizes non-radiative recombination. One example of a core-shell nanoparticle is ZnS grown on the surface of CdSe cores. FIG. 1B schematically depicts a core-shell nanoparticle 140 with a core 150 including InP and a shell 160 including ZnS.
Another approach is the formation of a core-multi shell structure where the electron-hole pair is completely confined to a single shell layer. In these structures, the core is of a wide bandgap material, surrounded by a thin shell of narrower bandgap material, and capped with a further wide bandgap layer, such as CdS/HgS/CdS. In such a structure, a few monolayers of mercury sulfide (HgS) are formed on the surface of the core CdS nanocrystal and then capped by additional CdS. The resulting structures exhibit clear confinement of photo-excited carriers in the narrower bandgap HgS layer. FIG. 1C schematically depicts a multi-shell nanoparticle 170 with a core 180 including InP, a shell 190 including ZnSe, and an outer shell 195 including ZnS. FIG. 2 schematically depicts a nanoparticle 200 coated with a capping layer 210 having a head group 220 (bonded to the nanoparticle) and hydrocarbon chains 230.
The outermost layer of organic material (i.e., the capping agent) or sheath material helps to inhibit particle aggregation, and further protects the nanoparticle from the surrounding chemical environment. It also may provide a means of chemical linkage to other inorganic, organic, or biological material. In many cases, the capping agent is the solvent in which the nanoparticle preparation is undertaken, and consists of a Lewis base compound or a Lewis base compound diluted in a inert solvent such as a hydrocarbon. The capping agent includes a lone pair of electrons that are capable of donor-type coordination to the surface of the nanoparticle, and may include mono- or multi-dentate ligands of the types: phosphines (trioctylphosphine, triphenolphosphine, t-butylphosphine), phosphine oxides (trioctylphosphine oxide), alkyl phosphonic acids, alkyl-amine (hexadecylamine, octylamine), aryl-amines, pyridines, long chain fatty acids, and thiophenes. Other types of materials may also be appropriate capping agents.
The outermost layer (capping agent) of a quantum dot may also consist of a coordinated ligand that processes additional functional groups that can be used as chemical linkage to other inorganic, organic or biological material. In such a case, the functional group may point away from the quantum dot surface and is available to bond/react with other available molecules, such as primary, secondary amines, alcohols, carboxylic acids, azides, or hydroxyl groups. The outermost layer (capping agent) of a quantum dot may also consist of a coordinated ligand, processing a functional group that is polymerizable, which may be used to form a polymer around the particle.
The outermost layer (capping agent) may also consist of organic units that are directly bonded to the outermost inorganic layer, and may also process a functional group, not bonded to the surface of the particle, that may be used to form a polymer around the particle.
Important issues related to the synthesis of high-quality semiconductor nanoparticles are particle uniformity, size distribution, quantum efficiencies, long-term chemical stability, and long-term photostability. Early routes applied conventional colloidal aqueous chemistry, with more recent methods involving the kinetically controlled precipitation of nanocrystallites, using organometallic compounds.